Try to implement sphere self obstruction inside

Try to implement sphere self obstruction inside

The sphere was completely lit inside by the lights outside.
I cannot stop ignoring at least parts of the objects else planes would
render with shadow acne.
So I tried to ignore the object if it obstructs itself very close from
the rendered place - this should prevent shadow acne.
The sphere however ignores the distant intersection when the close one
needs to be ignored. This manifests as "light acne" - some part are
correctly in shadow, while others cause the object to be ignored and
light passes through.